This invention relates to a solder reflow furnace for use in the soldering of electronic components to printed circuit boards with a solder paste. It also relates to a panel-type blower for use in such a reflow furnace.
When soldering of electronic components to a printed circuit board is carried out by heating the printed circuit board in a solder reflow furnace, it is necessary to heat all portions of the printed circuit board, including its side edges, corners and central portions at a uniform heating rate so that a uniform temperature profile is obtained. It is desirable that all the portions of the printed circuit board have substantially the same temperature profile and that all the solder paste, which is applied to the printed circuit board prior to heating, reflow simultaneously without there being portions where soldering does not take place or where overheating occurs.
Conventional solder reflow furnaces commonly use infrared heaters to heat printed circuit boards. However, when the packing density of printed circuit boards is very high, it is impossible to heat the printed circuit boards uniformly using only infrared panel heaters. This is because the radiation emitted from the infrared panel heaters travels in straight lines and can not reach components which lie in the shadow of other components. As a result, cold areas occur on the printed circuit board where the temperature is not high enough for soldering to take place. On the other hand, tall electronic components come so close to infrared panel heaters that the top portions of the components are sometimes overheated, resulting in thermal damage.
Another conventional type of reflow furnace, referred to as a hot gas reflow furnace, uses a heated gas to heat printed circuit boards to a soldering temperature. Hot gas reflow furnaces do not have the problems of reflow furnaces using infrared heaters and therefore have come to be widely used.
Hot gas reflow furnaces are classified as either the downward blowing type or the upward and downward blowing type.
In the downward blowing type, a blower for hot gas is installed in the upper portion of a tunnel of a furnace above a conveyor for transporting printed circuit boards through the tunnel. See Japanese Patent Publication No. 38985/1986, Japanese Patent Application Laid-Open Specification No. 177960/1988, Japanese Utility-Model Publication No. 23666/1989, Japanese Utility-Model Application Laid-Open Specification No. 177076/1989, and so on.
In the upward and downward blowing type, blowers for hot gas are installed in a tunnel both above and below a conveyor.
Reflow furnaces of the downward blowing type are very effective for uniformly heating a one-sided printed circuit board (one having electronic components mounted on only one of its sides), since hot gas is blown against only the upper surface of the printed circuit board. However, downward blowing furnaces are less suitable for heating a two-sided printed circuit board (one having electronic components mounted on both of its sides), since hot gas from the blower does not reach the bottom surface of the printed circuit board. As a result, the bottom surface is heated less than the top surface, and the temperature profile of the bottom surface is different from that of the top surface.
Reflow furnaces of the upward and downward blowing type can simultaneously blow hot gas onto both sides of a printed circuit board. Since such furnaces are suitable for heating two-sided printed circuit boards, they are widely used.
Reflow furnaces of the upward and downward blowing type can be further divided into the following two types.
(1) The first type comprises furnaces in which an upper blower and a lower blower are disposed opposite one another such that the gas discharge port of each blower opposes the gas discharge port of the opposing blower and such that the gas inlet of each blower opposes the gas inlet of the opposing blower. See Japanese Patent Application Laid-Open Specification No. 180368/1985, Japanese Patent Application Laid-Open Specification No. 186270/1989, Japanese Patent Application Laid-Open specification No. 278965/1989, and so on. Furnaces of this type are referred to as "opposing flow furnaces".
(2) Furnaces in which an upper blower and a lower blower are disposed opposite one another such that the gas discharge port of each blower opposes the gas inlet of the opposing blower. See Japanese Patent Application Laid-Open Specification No. 83395/1989, for example. Furnaces of this type are referred to as "circular flow furnaces".
In an opposing flow furnace, hot gases blown from the upper and lower blowers run into each other midway between the two blowers. Thus, when a printed circuit board passes between the upper and lower blowers, the top and bottom surfaces of the printed circuit board are equally heated by the hot gases blown from the blowers. However, when no printed circuit board is present between the blowers, hot gases from the upper and lower blowers are locally mixed in a very limited area, resulting in an unstable temperature distribution within the furnace.
In reflow furnaces of the hot gas blowing type, it is generally desirable that hot gases at the same temperature strike against both sides of a printed circuit board. Since the temperature distribution in the longitudinal direction in an opposing flow furnace is unstable in the absence of a printed circuit board, it is impossible to perform uniform heating when a printed circuit board is introduced into the area between opposing blowers.
In a circular flow furnace, hot gases from both the upper and lower blowers circulate within the furnace to create a uniform temperature distribution within the furnace whether or not a printed circuit board is present in the furnace. However, when a printed circuit board passes between the upper and lower blowers, the flow of gas between the blowers is interrupted by the printed circuit board, and only one side of the printed circuit board is heated at a time. As a result, the printed circuit board can not be uniformly heated. The nonuniformity of heating is particularly great when the gas discharge port of one blower is positioned remote from that of the opposing blower in the longitudinal direction of the furnace. This is because the two sides of a printed circuit board passing through such a furnace are heated at different times rather than simultaneously. Thus, within the furnace, there are heating zones where only the top surface of a printed circuit board is heated and other zones where only the bottom surface is heated.
Thus, in conventional circular flow furnaces it is necessary to install blowers such that the discharge ports for hot gas are close to each other and such that numerous circulating paths are established in a limited area within the furnace. This requires that the height of the furnace be as great as possible, resulting in a substantial loss of heat.